Re: [PATCH v9 1/3] xen/domain: unify domain ID allocation

[Julien Grall <julien@xxxxxxx>]

Hi Denis,

On 06/06/2025 07:55, dmkhn@xxxxxxxxx wrote:
> On Thu, Jun 05, 2025 at 10:58:48PM +0100, Julien Grall wrote:
> > > +        if ( domid == DOMID_INVALID )
> > > +            panic("Error allocating ID for domain %s\n", dt_node_name(node));
> > > +
> > > +        d = domain_create(domid, &d_cfg, flags);
> > >            if ( IS_ERR(d) )
> > >                panic("Error creating domain %s (rc = %ld)\n",
> > >                      dt_node_name(node), PTR_ERR(d));
> > > diff --git a/xen/common/domain.c b/xen/common/domain.c
> > > index abf1969e60..ae0c44fcbb 100644
> > > --- a/xen/common/domain.c
> > > +++ b/xen/common/domain.c
> > > @@ -66,6 +66,10 @@ DEFINE_RCU_READ_LOCK(domlist_read_lock);
> > >    static struct domain *domain_hash[DOMAIN_HASH_SIZE];
> > >    struct domain *domain_list;
> > >
> > > +/* Non-system domain ID allocator. */
> > > +static DEFINE_SPINLOCK(domid_lock);
> > > +static DECLARE_BITMAP(domid_bitmap, DOMID_FIRST_RESERVED);
> > > +
> > >    /*
> > >     * Insert a domain into the domlist/hash.  This allows the domain to be 
> > > looked
> > >     * up by domid, and therefore to be the subject of hypercalls/etc.
> > > @@ -1449,6 +1453,8 @@ void domain_destroy(struct domain *d)
> > >
> > >        TRACE_TIME(TRC_DOM0_DOM_REM, d->domain_id);
> > >
> > > +    domid_free(d->domain_id);
> > > +
> > >        /* Remove from the domlist/hash. */
> > >        domlist_remove(d);
> > >
> > > @@ -2405,6 +2411,54 @@ domid_t get_initial_domain_id(void)
> > >        return hardware_domid;
> > >    }
> > >
> > > +domid_t domid_alloc(domid_t domid)
> > > +{
> > > +    spin_lock(&domid_lock);
> > > +
> > > +    if ( domid < DOMID_FIRST_RESERVED )
> > > +    {
> > > +        if ( __test_and_set_bit(domid, domid_bitmap) )
> > > +            domid = DOMID_INVALID;
> > > +    }
> > > +    else
> > > +    {
> > > +        static domid_t domid_last;
> > > +        /* NB: account for late hwdom case, skip ID#0 */
> > I am somewhat confused with this comment. For the late hwdom case, I
> > thought we were using a non-zero ID. Dom0 should also always be the
> > first dom0 to be reserved. Can you clarify?
> My current understanding is:
> - the ID of "domain 0" (privileged domain) can be overridden at the boot-time
>    via hardware_domid parameter.
> > - there's only one reserved (and configurable) domain ID == 
hardware_domid in
>    the allocation range (which is 0 by default).
> > - get_initial_domain_id() returns the reserved domain ID value 
(which is
>    used in the in the follow on change to keep the change isolated).
>
> Is my understanding correct?
I have replied yesterday night on a separate thread about this behavior 
[1]. Rather than duplicating it, I would suggest to move the 
conversation there.
In short, I believe the late domain support was recently broken.

Cheers,

[1] 
https://lore.kernel.org/xen-devel/20250528225030.2652166-1-dmukhin@xxxxxxxx/T/#mdcdf3802a913859243ff6ce841
445cfab265145f

--
Julien Grall